I've always liked 'em, as long as they don't get too distracting. Besides, it makes it much quicker to determine who wrote a post if you're too lazy to read someone's name, like I am.

If you don't want big, distracting avatars then come to a compromise and limit the size drastically. I mean REALLY drastically - a tiny little piclet in the upper left corner that brings the words "Gameboy Sprite" to mind. I think that has a nice asthetic appeal, myself.